Low pressure injection and turbulent mixing in selective catalytic reduction system

Abstract

A selective catalytic reduction system for engine exhaust injects a source of ammonia, preferably an aqueous urea solution, or other ammonia solution, to preferably evaporate, decompose and hydrolyze to produce ammonia to react with and reduce NOx in the exhaust. A turbulence generator between the injector and a downstream catalyst enhances ammonia mixing upstream of the catalyst. A perforated reflector between the injector and the catalyst passes the exhaust through the perforations and reflects exhaust back toward the injector, to generate the turbulence to achieve enhanced ammonia mixing. A low pressure generator creates a low pressure evaporative diffusion enhancement zone at the injector accelerating evaporation rate and accelerating ammonia diffusion and mixing with the exhaust.

What is claimed is:  
     
       1. A selective catalytic reduction device for engine exhaust comprising: 
       a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust;  
       an injector in said housing for injecting a source of ammonia to react with and reduce NO x  in said exhaust;  
       a catalyst in said housing downstream of said injector;  
       a turbulence generator in said housing between said injector and said catalyst and enhancing ammonia mixing upstream of said catalyst,  
       wherein said turbulence generator comprises a perforated reflector, said perforated reflector comprises a bowl-shaped perforated member having an upstream side with a concave surface facing said injector, said bowl-shaped perforated member has a plurality of perforations through which exhaust flows, and wherein said concave surface extends between said perforations and reflects exhaust toward said injector, and said perforations are triangular.  
     
     
       2. A selective catalytic reduction device for engine exhaust comprising: 
       a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust;  
       an injector in said housing for injecting a source of ammonia to react with and reduce NO x  in said exhaust;  
       a catalyst in said housing downstream of said injector;  
       a turbulence generator in said housing between said injector and said catalyst and enhancing ammonia mixing upstream of said catalyst,  
       wherein said turbulence generator comprises a perforated reflector, said perforated reflector comprises a bowl-shaped perforated member having an upstream side with a concave surface facing said injector, said bowl-shaped perforated member has a plurality of perforations through which exhaust flows, and wherein said concave surface extends between said perforations and reflects exhaust toward said injector, and said concave surface is a portion of a sphere.  
     
     
       3. The selective catalytic reduction device according to  claim 2  wherein said concave surface is a hemisphere. 
     
     
       4. The selective catalytic reduction device according to  claim 2  wherein said injector has an injection tip at the center of said sphere. 
     
     
       5. The selective catalytic reduction device according to  claim 2  wherein said housing extends axially along an axis and exhaust flows generally axially through said housing, and wherein said injector and the center of said sphere are on said axis. 
     
     
       6. The selective catalytic reduction device according to  claim 2  wherein said source of ammonia is an aqueous urea solution which evaporates, decomposes and hydrolyzes to produce ammonia to react with and reduce NO x  in said exhaust. 
     
     
       7. The selective catalytic reduction device according to  claim 2  wherein said source of ammonia is an ammonia solution. 
     
     
       8. A selective catalytic reduction device for engine exhaust comprising: 
       a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust;  
       an injector in said housing for injecting a source of ammonia to react with and reduce NO x  in said exhaust;  
       a catalyst in said housing downstream of said injector;  
       a turbulence generator in said housing between said injector and said catalyst and enhancing ammonia mixing upstream of said catalyst,  
       wherein said turbulence generator comprises a perforated reflector, said perforated reflector comprises a bowl-shaped perforated member having an upstream side with a concave surface facing said injector, said bowl-shaped perforated member has a plurality of perforations through which exhaust flows, and wherein said concave surface extends between said perforations and reflects exhaust toward said injector, and said concave surface is parabolic as defined by a curve generated by a point moving so that its distance from a fixed point at said injector is equal to its distance from a fixed plane downstream of said concave surface.  
     
     
       9. The selective catalytic reduction device according to  claim 8  wherein said housing extends axially along an axis and exhaust flows generally axially through said housing, and wherein said fixed plane is perpendicular to said axis. 
     
     
       10. The selective catalytic reduction device according to  claim 9  wherein said injector is on said axis. 
     
     
       11. The selective catalytic reduction device according to  claim 10  wherein said injector injects said source of ammonia along a spray angle relative to said axis, said parabolic surface has a parabolic surface height which is defined as the distance along said axis from said injector to the center of said parabolic surface, and said parabolic surface height is a decreasing function of said spray angle. 
     
     
       12. The selective catalytic reduction device according to  claim 8  wherein said source of ammonia is an aqueous urea solution which evaporates, decomposes and hydrolyzes to produce ammonia to react with and reduce NO x  in said exhaust. 
     
     
       13. The selective catalytic reduction device according to  claim 8  wherein said source of ammonia is an ammonia solution. 
     
     
       14. A selective catalytic reduction device for engine exhaust comprising: 
       a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust;  
       a low pressure generator in said housing creating a low pressure diffusion enhancement zone;  
       an injector in said housing injecting a source of ammonia in said low pressure diffusion enhancement zone to react with and reduce NO x  in said exhaust, said low pressure diffusion enhancement zone accelerating mixing of said ammonia with said exhaust;  
       a catalyst in said housing downstream of said injector;  
       and further comprising in combination a turbulence generator in said housing downstream of said injector and enhancing mixing and transport of ammonia,  
       wherein said turbulence generator comprises a perforated reflector, said perforated reflector comprises a bowl-shaped perforated member having an upstream side with a concave surface facing said injector, a downstream side with a convex surface facing said catalyst, and a plurality of perforations through which exhaust flows, and wherein said concave surface extends between said perforations and reflects exhaust toward said injector, and said concave surface is a portion of a sphere, said housing extends axially along an axis and exhaust flows generally axially through said housing, said injector has a downstream injection tip on said axis and at the center of said sphere.  
     
     
       15. The selective catalytic reduction device according to  claim 14  wherein said source of ammonia is an aqueous urea solution which evaporates, decomposes and hydrolyzes to produce ammonia to react with and reduce NO x  in said exhaust. 
     
     
       16. The selective catalytic reduction device according to  claim 14  wherein said source of ammonia is an ammonia solution. 
     
     
       17. A selective catalytic reduction device for engine exhaust comprising: 
       a housing having an upstream inlet for receiving said exhaust, and a downstream outlet for discharging said exhaust;  
       a low pressure generator in said housing creating a low pressure diffusion enhancement zone;  
       an injector in said housing injecting a source of ammonia in said low pressure diffusion enhancement zone to react with and reduce NO x  in said exhaust, said low pressure diffusion enhancement zone accelerating mixing of said ammonia with said exhaust;  
       a catalyst in said housing downstream of said injector;  
       and further comprising in combination a turbulence generator in said housing downstream of said injector and enhancing mixing and transport of ammonia,  
       wherein said turbulence generator comprises a perforated reflector, said perforated reflector comprises a bowl-shaped perforated member having an upstream side with a concave surface facing said injector, a downstream side with a convex surface facing said catalyst, and a plurality of perforations through which exhaust flows, and wherein said concave surface extends between said perforations and reflects exhaust toward said injector, and said concave surface is parabolic as defined by a curve generated by a point moving so that its distance from a fixed point at said injector is equal to its distance from a fixed plane downstream of said concave surface, said housing extends axially along an axis and exhaust flows generally axially through said housing, said fixed plane is perpendicular to said axis, and said injector is on said axis.  
     
     
       18. The selective catalytic reduction device according to  claim 17  wherein said injector has a downstream injection tip, and said low pressure generator comprises a plurality of curved fins upstream of said injector tip and creating a swirl in exhaust flow which swirl creates said low pressure diffusion enhancement zone downstream of said fins, and wherein said fins have a downstream edge diverging from said axis along a fin angle relative to said axis, said parabolic surface has a parabolic surface height which is defined as the distance along said axis from said injector to the center of said parabolic surface, and said parabolic surface height is a decreasing function of said fin angle. 
     
     
       19. The selective catalytic reduction device according to  claim 17  wherein said source of ammonia is an aqueous urea solution which evaporates, decomposes and hydrolyzes to produce ammonia to react with and reduce NO x  in said exhaust. 
     
     
       20. The selective catalytic reduction device according to  claim 17  wherein said source of ammonia is an ammonia solution.